X_CO (left column) and X_CI (right column) conversion factors vs. metallicity as a function of the FUV intensity (top row), the cosmic-ray ionization rate (middle row), and the density distribution (bottom row). The panels in the top row illustrate the dependence on ζ_CR for a fixed FUV intensity of χ/χ₀ = 1. The panels in the middle row show the dependence on χ/χ₀ for a fixed ζ_CR = 10⁻¹⁷ s⁻¹. In the top and middle rows, we consider all three m and three σ values (nine distributions in total) of Table 2. The panels in the bottom row illustrate the dependence on the mean, m, of the A_V-PDF for a fixed σ_PDF = 0.4. The bottom panels are averages for 10⁻¹⁷ ≤ ζ_CR ≤ 10⁻¹⁵ s⁻¹ and 0.5 ≤ χ/χ₀ ≤ 10². In all panels, the gray scatter points and the gray lines correspond to the observations and best-fit relations described in Fig. 6.